using System.Collections.Generic;

namespace System.Threading.Tasks {
	// Provides a task scheduler that ensures a maximum concurrency level while 
	// running on top of the thread pool.
	internal sealed class LimitedConcurrencyLevelTaskScheduler : TaskScheduler {
		// Indicates whether the current thread is processing work items.
		[ThreadStatic]
		private static bool _currentThreadIsProcessingItems;

		// The list of tasks to be executed 
		private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks)

		// The maximum concurrency level allowed by this scheduler. 
		private readonly int _maxDegreeOfParallelism;

		// Indicates whether the scheduler is currently processing work items. 
		private int _delegatesQueuedOrRunning = 0;

		// Creates a new instance with the specified degree of parallelism. 
		public LimitedConcurrencyLevelTaskScheduler(int maxDegreeOfParallelism) {
			if (maxDegreeOfParallelism < 1) throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
			_maxDegreeOfParallelism = maxDegreeOfParallelism;
		}

		// Queues a task to the scheduler. 
		protected sealed override void QueueTask(Task task) {
			// Add the task to the list of tasks to be processed.  If there aren't enough 
			// delegates currently queued or running to process tasks, schedule another. 
			lock (_tasks) {
				_tasks.AddLast(task);
				if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism) {
					++_delegatesQueuedOrRunning;
					NotifyThreadPoolOfPendingWork();
				}
			}
		}

		// Inform the ThreadPool that there's work to be executed for this scheduler. 
		private void NotifyThreadPoolOfPendingWork() {
			ThreadPool.UnsafeQueueUserWorkItem(_ => {
				// Note that the current thread is now processing work items.
				// This is necessary to enable inlining of tasks into this thread.
				_currentThreadIsProcessingItems = true;
				try {
					// Process all available items in the queue.
					while (true) {
						Task item;
						lock (_tasks) {
							// When there are no more items to be processed,
							// note that we're done processing, and get out.
							if (_tasks.Count == 0) {
								--_delegatesQueuedOrRunning;
								break;
							}

							// Get the next item from the queue
							item = _tasks.First.Value;
							_tasks.RemoveFirst();
						}

						// Execute the task we pulled out of the queue
						TryExecuteTask(item);
					}
				}
				// We're done processing items on the current thread
				finally { _currentThreadIsProcessingItems = false; }
			}, null);
		}

		// Attempts to execute the specified task on the current thread. 
		protected sealed override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) {
			// If this thread isn't already processing a task, we don't support inlining
			if (!_currentThreadIsProcessingItems) return false;

			// If the task was previously queued, remove it from the queue
			if (taskWasPreviouslyQueued)
				// Try to run the task. 
				if (TryDequeue(task))
					return TryExecuteTask(task);
				else
					return false;
			else
				return TryExecuteTask(task);
		}

		// Attempt to remove a previously scheduled task from the scheduler. 
		protected sealed override bool TryDequeue(Task task) {
			lock (_tasks) return _tasks.Remove(task);
		}

		// Gets the maximum concurrency level supported by this scheduler. 
		public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }

		// Gets an enumerable of the tasks currently scheduled on this scheduler. 
		protected sealed override IEnumerable<Task> GetScheduledTasks() {
			bool lockTaken = false;
			try {
				Monitor.TryEnter(_tasks, ref lockTaken);
				if (lockTaken) return _tasks;
				else throw new NotSupportedException();
			}
			finally {
				if (lockTaken) Monitor.Exit(_tasks);
			}
		}
	}
}
